Glass that has been subjected to a chemical strengthening process such as ion exchange (hereinafter, such glass will also be referred to as “chemically strengthened glass”) has been used as a cover glass of display devices such as digital cameras, cell phones, and PDAs (Personal Digital Assistants), and as a glass substrate of displays. Since the chemically strengthened glass has higher mechanical strength than the case of unstrengthened glass, the chemically strengthened glass is preferable for use in these applications.
The chemical strengthening process by an ion-exchange is a process by which the metal ions (for example, Na ions) having a smaller radius contained in glass are replaced by metal ions (for example K ions) having a larger radius contained in the glass to generate a compressive stress layer on a glass surface, and improve the glass strength.
On the other hand, chemically strengthened glass is sometimes required to have functions such as reflection prevention, photoabsorption, and transparent conductivity, and these functions are particularly desired in the cover glass and display.
Such functions can be given to glass by forming a functional film on a glass substrate surface. As the method of forming such a functional film, a sol-gel method, in which a coating liquid containing fine particles is applied to a glass substrate, followed by drying and calcining to thereby perform gelling, thereby forming a functional film, is known (Patent Documents 1 and 2).
For chemical strengthening of a glass substrate by an ion-exchange, wherein a functional film has been formed on the surface of the glass substrate, the functional film is necessary to have channels for passage of Na ions, K ions and the like, and thus, the functional film needs to be porous intentionally for this purpose. In Patent Documents 1 and 2, the functional film is made porous by containing hollow particles in the functional film, by using the sol-gel method.
Patent Document 1 discloses a method for producing a chemically strengthened glass article, in which a coating liquid containing inorganic fine particles and a hydrolytic metallic compound is applied onto at least one surface of a glass substrate, followed by hearing to form a functional film covering the glass substrate surface, and then, subjecting to chemical strengthening.
Patent Document 2 discloses a method for producing antireflective strengthened glass, in which an antireflective film containing a hydrolytic condensation product of a silicon compound, a silica sol having inner cavities, and a metal chelate compound is formed on a surface of a glass substrate, and the glass substrate with the antireflective film is subjected to chemical strengthening process based on an ion-exchange.